There is a very easy solution for your problem , even i had the same problem and now it is fixed.
Go to control panel programs and features and uninstall a software called Smartbytes.
This will solve your crackling issue completely.
I also have the exact same issue with my inspiron 5570 and have tried everything to resolve it but with no success. - Changing drivers, SmartByte, different AC adaptor and many, many other fixes that I read about. Some seemed to work temporarily but none provided a permanent fix.
Like others the issue is only present when the laptop is plugged in.
The only thing I could do to resolve it was to use a bluetooth receiver to connect to my speakers but the issue there is that it has high latency so the audio on videos is very out of sync.
Really is quite maddening that such an obvious issue got through QC and that Dell have not been able to resolve it.